Peak limiting amplifier



Oct- 12, 1943- L. P. TUCKERMAN 2,331,360

PEAK LIMITING AMPLIFIER 'l FiledAug. 18, 1942 PatentedOt. 12, 1943UNITED STATES PATENT OFFICE PEAK LIMITING AMPLIFIER Lucien P.Tucker-man, washington, D, c., assignor to Federal Telephone and RadioCorporation, a corporation of Delaware Application August 18, 1942,Serial No. 455,19l

Claims.

This invention relates to' ampliers of the automatic volume controltype, and more particularly to such amplifiers including a pushpullstage of amplification.

While automatic volume control circuits in combination with ampliers ofthe kind referred matic volume control circuit. -Since the effectiveamplification or output of the two tubes, as purchased in the openmarket, or as obtained within Y the `limits vof commercial tolerancesfor such to. are in general very old, the available forms of suchcircuits appear to be subject to quite rigid limitations which cannot beovercome by ordinary means without undesirably affecting the stabilityof operation of the circuits. Diihculties have been found in cases wherethe gain of the y amplifier` is veryv high, or where a very high qualityof wave transmission is required over va wide frequency range, or wherethe speed of tubes, is generally unequal when the tubes are operation,an undesirable amount` of experimentation, extra skill, effort, ortimemay be or interruption of transmission. For example, in

the case of a relatively high gain amplifier employing a push-pullamplifying stage and having the usual volume control circuit fortransmitting a rectified impulse from the output of the amplier toan'input coupling common to the two input circuits of the push-pullstage, it has been found that a sudden large increase of amplitude ofthe signarwave, may entirely paralyze the operation of the amplier for abrief period at least, or may cause self-oscillation of the amplifier,or may otherwise interfere with the normal intended operation of theamplifier.

Some ofthe objects of the present invention are to counteract one ormore of the above dif-l culties, to provide an improved wide frequency,

. replacements of tubes inpush-pull amplifiers of the automatic volumecontrol type .referred to above, where any two tubes generallydlifersomewhat in their characteristics and thus interfere with theproduction of a perfect balance in the push-pull stage, I have foundthat it is important to position the tubes in the push-pull stage in Isuch relation to each other that the resulting unbalance of the circuithas an eiect whose phase is in degenerative relationwith theautorequired in trying to make the amplifier operate vwith. suitablestability under theconditions re- Y ferred to. f

In accordance with the present invention, these difficulties may becounteracted eectively and the range of stable operation of theamplii'ler extended by providing an auxiliary unbalance of suitableamount and suitable phase in the transmission circuit which includes thepush-pull amplifier and extends from' the volume control input terminalsofthe amplifier to the rectifier in the automatic volume controlcircuit. This unbalancing means may, for example, consist of in thepush-pull stage, to reduce the effective amplification of one side ofthe push-pull amplifier, so that the resulting unbalanced characteristicis greater than the unbalanced transmission produced by the inherentpredetermined' difference in the tubes themselves, and the phaserelation of the unbalanced transmission characteristic is such as toproduce degeneration with respect to the feedback path formed by theautomaticvolume control circuit. 'I'he resulting amplifier is adapted toquickly and efficiently limit peak amplitudes in the, amplied signalwave.

These and other objects and features of the invention will be understoodmore clearly from the following detailed description in connection withlthe accompanying drawing and the ap- 'pended claims.

The drawing shows a schematic circuitdiagram of an amplier illustrativeof one form of the present invention.

In the drawing, the amplier I, includes a push-pull amplifying stage 2,and the extra stages 3, the push-pull stage including amplifying units'4 and 5 which are shown, by way of example,vas being of the electrondischarge type, although it will be understood. that the invention isapplicable to amplifying units of other types.

' symmetry and circuit balance. The input output terminal 38 foramplifier 35.

' duced,

`The secondary Each electrondischarge device includes an anode, anelectron emitting cathode, and an electron control element.

The source of waves I impresses signal 4waves to be amplified on theprimary of transformer I I, the secondary of which has a circuit inshunt therewith, including resistance elements I2, I3, I4 and I5 inseries', the resistance I3 being made equal to resistance `I4, andresistance I2 being made equal to resistance I for the purpose of signalvoltage across resistance I3, is transmitted to the inputterminals I8,I1 of amplifying unit 4, and input signal voltage across resistance I4,is transmitted to input terminals I8 and I9 of amplifying unit 5,terminals I1 and I9 being connected together at ground 2U. Terminal I5connects directly with the control grid of unit 4, and terminal I8connects directly with the control grid of unit 5, the terminals I6 andIB constituting push-pull input terminals for the push-pull amplifierstage 2. The grid-cathode circuit for each unit 4 and 5 has avportion incommon including y the resistance coupling 2| having the automaticvolume control terminals BI and 62, the condenser 63 being in shunttherewith for opposing too sudden changes in the automatic volumecontrol grid bias.

The resistance 22 is connected between the cathode of unit 4 and ground20, and the corresponding resistance 23 is connected between'the cathodeof unit 5 and ground 20, resistances 22 and 23 serving in well knownmanner to provide a normal negative biasing potential for the respectivecontrol grids of units 4 and 5. Space current is suppliedto the anodesof units 4 and 5, through the coupling resistances 24 and 25. Outputcurrent from the anodes of units 4 and 5 is transmitted through therespective direct current blocking condensers 28, 21 to the -outputterminals 28, 29 across which is connectedin series, the resistor 30,auto-transformer coil 3l,

and the resistance 32, the mid-point 33 of4 the coil 3l being grounded.Amplified output current passing through circuit 83, 3|, 32 transmitsiinpulses to the control grid amplifier of the electron discharge typehaving a capacity-resistance coupling beween its anode andthe control ofamplifier 35 of the electron discharge grid which connects with outputtype, the anode of terminal 38, to which is connected one terminal oftheprimary of transformer 31, the other terminal of said primary being.connected with the It will be seen that terminals 38 and 38 serve asoutput terminals for the amplifier I. The number of extra amplifierstages 3 may obviously be varied to suit requirements as indicated bythe dotted line 39, at which point additional of ampliiication not shownmaybe introand similarly itv willl be apparent that additional push-pullstages of amplification similar to push-pull stage 2, may be introducedimmediately following stage 2 atthe points indicated the dotted lines48. The output'transformer 31, may obviously have its terminals 38, 38,connected directly with terminals 28, 2 8, when the number of extrastages 3 are reduced to zero. winding 4I, of 4transformer 31,is-connected with the load 42, which may be any desired form ofutilization circuit for the amplined signal waves. The secondary winding43 of transformer 31, is coupled with rectifier 44, the direct currentoutput conductors 45 of the rectiner being connected directly with theterminals stages 6I, 62 of the coupling resistance 2| constituting theautomatic volume control input terminals for the amplifier.

Rectifier 44, preferably includes a full-wave rectifier tube 46 havingtwo cathodes 41 connected respectively with opposite terminals of coil43. 'I'he anodes 48 being connected together with one of the conductors45, the other conductor 45 being connectedrthrough potentiometerresistances 49, by way of the adjustable tap 53 with the mid-point ofcoil 43. Potentiometer 49 is connected across the direct current sourceof potential 5I, poled in a 'direction to oppose the output potential ofthe rectier. Adjustment of potentiometer 49, permits of adjustment ofthe threshold amplitude of amplied output signals below which norectification takes place in automatic volume control circuit extendingfrom coil 43 to coupling 2|, and below which' no volume control impulsesare transmitted to the input of the amplier, this operation being lnaccordance with the well-known practice for limiting the 'operation ofthe automatic volume' control to apply to only those Iamplified outputsignal amplitudes which exceed a predetermined value. The capacity 63 ismade small enough to permit of a relatively quick application of theautomatic volumel control biasing potential to the grids of amplifyingunits 4 and 5, when the amplitude of the output signal waves tend toexceed the desired value. The variable resistance 52 may be included inone of the conductors 45 for damping the volume control impulses.

When the requirements of the amplifier are such that a very high gain ofamplification is to be obtained, or where a very wide range of signalfrequencies are to be amplied, or where a relatively quick response isto be made. by the automatic volume control circuit and especially wheretwo or more of these conditions are to be met in a given amplifier, Ihave found that small unbalances in the push-pull stage 2 tend to causeserious ,dinicultiea 'I'hese difllculties are apparently due in part toinherent differences between the characteristics of available amplifiertubes for substitution in units 4 and 8. These tube differences producean unbalance which tends to cause instability .of operation of theamplifier and may momentarily paralyze operationv of the amplifier orcause it to produce self-oscillations.

or cause the production of peak amplitudes momentarily far in excess ofthe limit set by the automatic volume control circuit, or may otherwiseinterfere with the stable operation ofthe ampliiler when there is asudden increase inthe amplitude of signal waves from the source I8.andothe amplitude reaches a value sumciext i sense with vrespect to theautomatic volume control or feedback circuit 45. The resultingstabilization of operation serves to limit peak ampli- .tudessatisfactorily to the maximum set by the automatic volume controlcircuit 45.

It will be apparent that when the number of stages of amplification ischanged from that shown in the drawing, or when the phase of any tocause operation of the automatic control ciruitf the other side of thevcircuit so that the unbalance of the circuit is reversed 180 in phase,in order to produce degeneration around the closed path formed by theamplifier in combination with the automatic volume control circuit 45.

The value of the resistance 60, or other unbalancing means introducedinone side of the transmission circuit, referred to above is preferablynot less than that necessary to compensate for any undesired unbalancedue to inherent differences between amplifying units of the same typeutilized in the push-pull stage 2, and .is preferably of sulcierlt valueto produce a safe margin of unbalance in excess-of that necessary tocompensate for said tube or amplifying unit dierences, so thati thedesired stability is achieved irrespective of the relative positions ofthe unequal amplifying units or tubes in the push-pull stage 2.

The invention is not limited to the use of ampliers of the space currenttype in the pushpull stage 2, but is applicable to various other typesof ampliers, and the form of the unbalancing 'means and its position inthe transmission circuit may have any ofa wide range of variationswithout departing from the invention as set forth in the appendedclaims. l

What is claimed `is:-

1. In an amplifying system having a push-pull amplifying stage, whereintwo amplifying umts have individual input circuits providing pushportionof each said input circuit being in common and including a couplinghaving automatic volume controlv input terminals, anda feedbackcircuitincluding rectifying means responsive to an amplified wave derived fromthe outputpf said push-pull stage and connected with s/aid volumevIcontrol input terminals for transmitting volume control impulsesthereto, the combination wherein there is provided a transmissioncircuit including said push-pull stage and extending from said volumecontrol input terminals to said rectifying meanssaidtransmission'circuit includingpredetermined circuit unbalancing means 4.In an amplifying system according to claim 1, the combination whereinsaid rectifying means includes a rectifier of the full-wave type fortransmitting rectied impulses to said volum control input terminals.

5. In an amplifying system according to claim 1, the combination whereinsaid rectifying means includes a rectifier of the full-wave type fortransmitting rectified impulses to said volume control input terminals,a source of potential being provided in said feedback circuit betweensaid rectier and said volume control input terminals in opposition tothe output of said rectifier.

stage including two amplifying units having individual input circuitsvproviding push-pull input terminals for signals to be amplified, aportion f each said input circuit being in common and but having aninherent predetermined difference in characteristics whereby there isproduced an unbalanced transmission characteristic for impull inputterminals for signals to be amplified, a

for producing a circuit unbalance not less than 1 opposite sides of`said transmission circuit, the. y

resistancesof said elements being substantially different from eachother, the dinerence between saidresistances constituting said circuitunbalancingmeans. 'A

3L In an amplifying system according to claim 1, the combination whereineach of said amplifying umts includes an electron-discharge devicehaving an anode, a cathode, an electron controlelement, andananode-cathode circuit external] ing a diierence ofresis r to saiddevice, said anode-cathode circuits havpulses applied tosaid volumecontrol terminals,

`and auxiliary transmission Aunbalancing means responsive to wavestransmitted to said amplier,

corresponding conductor of greater resistance in the other side of saidcircuit.

8. An amplifier according to claim 6, wherein said amplifier includes acathode, an anode, and

electron control means, and" said auxiliary un.

balancing means includes means for making the normal resistance of theanode-cathode circuit of one of said amplifying units greater in valuethan that of the anode-cathode circuit of the other of saidamplifyingunits. I

9. An amplifier according to claim 6,'whereln the said auxiliaryunbalancing means includes a partially balanced circuit having aconductor of given resistance in one side of said circuit. and acorresponding conductor of greater resistance in the `other sidethereof, said rectifier being of the full-Wave type for Atransmittingrectified current to said volume control terminals.

10. An amplifier according to claim 6, wherein said auxiliaryunbalancing means includes a. partially balanced circuit having aconductor of given resistance in one side of said circuit anda icorresponding conductorof ldifferent resistance in the other sidethereof, said rectifier being of the full-wavetype for transmittingrectified current to said vvolume control terminals, said direct currentconnection including a sourceof direct current potential opposed to theoutput of said rectier.

, LUCIEN P. TUCKERMAN.

6. Anamplifler having a push-pull amplifying

